1. Field of the Invention
The present invention relates to a semiconductor device and a driving method thereof. The present invention relates to a semiconductor device including a memory and a processor.
Note that in this specification, a semiconductor device means a circuit having a semiconductor element (e.g., a transistor or a diode) and a device having the circuit. The semiconductor device also means any device that can function by utilizing semiconductor characteristics. For example, an integrated circuit, a chip including an integrated circuit, a display device, a light-emitting device, a lighting device, and an electronic device are all semiconductor devices.
2. Description of the Related Art
Increase in performance of CPUs (central arithmetic processing device: central processing unit) leads to advance in multifunction and improvement in performance of electronic devices such as a personal computer and a portable information terminal. This results in larger capacity of a main memory in a CPU. A main memory with large capacity is usually a DRAM (dynamic random access memory) in consideration of performance and cost.
Since a DRAM is a volatile memory, power needs to be supplied to retain data and refresh operation needs to be performed at regular intervals to fill the memory cell with electric charge. Therefore, increase in capacity of the main memory causes problems of increase in power consumption. As a technique of saving power of the DRAM, the combination of a volatile memory and a nonvolatile memory is proposed (see Patent Documents 1 and 2).
A typical example of a nonvolatile memory is a flash memory. Other examples of a known nonvolatile memory are a magnetoresistive random access memory, a ferroelectric random access memory, and a phase change memory. It is known that a memory having a memory cell with the same structure as a DRAM and having a transistor including an oxide semiconductor layer in the memory cell functions as a nonvolatile memory (see Patent Document 3).